Fear not, you penniless creatures! I'm one myself and allow me to explain my monthly finances.
I live in University of London intercollegiate accommodation that provides me with breakfast and dinner on weekdays, and brunch and dinner on weekends. The rent is about 505 pounds per week.
If you REALLY wanna be thrifty, you can spend as little as 60 pounds (excluding rent) per MONTH. Yes, I kid you not. In fact, aside from rent, I spent only 53.20 pounds in January. I have my meals in hall, indulge myself once in a while with a nice meal out, nick bread everyday for lunch, watch movies with my neighbours and go out with my friends. So despite spending only 60 pounds a month, rest assured I still have a social life, thank you.
However, if you don't wish to battle over books at the library, then on some months, especially at the start of terms, you may spend more poring through Amazon's webpages.
How much you ultimately spend really depends on you.